An industrial robot having a robot hand for carrying an object, such as a workpiece, is conventionally used for transferring the object, or for attaching an unmachined workpiece to an automatic machine tool and detaching a machined workpiece from the machine tool. In the conventional industrial robot, the rotating motion and the vertical linear movement of a robot body with respect to a stationary robot base as well as the telescopically extending and contracting motions of a robot arm with respect to the robot body are actuated by drive means comprised of electric motors, such as known electric servo motors, so that the robot hand attached to the free end of the robot arm by means of a robot wrist is brought toward and positioned at a desired position. However, since the robot wrist of the conventional industrial robot is generally so constructed that the rotating motion of the wrist about the central axis thereof as well as the swinging motion of the wrist about an axis perpendicular to the central axis is actuated by hydraulic or pneumatic actuating means, only fixed amounts of rotating and swinging motions of the wrist are permitted. That is, it is difficult to provide the robot wrist with a free unfixed desired amount of rotating and swinging motions. On the other hand, if an electric motor or motors were adopted for actuating the motions of the robot wrist, it would be possible to control the rotating and swinging motions of the robot wrist. However, provision of a mechanical convertor or convertors would always be required for the purpose of acquiring low speed rotating and swinging motions of the robot wrist from the high speed rotation of the electric motor or motors. Therefore, such mechanical convertor would have to be sufficiently small in size and simple in mechanism for enabling the robot wrist to move into a small space within a machine cooperating with the industrial robot thereby exhibiting a required performance of the industrial robot.